1. Field of the Invention
This invention relates to a position encoder for encoding the position of a moving member, and more particularly to a magnetic head of such a position encoder employing a magnetoresistive element.
2. Description of the Prior Art
It is known that, when a magnetoresistive element of material such as permalloy is placed in a magnetic field and a current is supplied thereto, the resistance value of the magnetoresistive element changes in accordance with the strength of the magnetic field. A transducer apparatus comprising a magnetic head formed by such magnetoresistive elements for detecting relative displacement of a magnetic information carrying medium is also commonly known from, for example, U.S. Pat. No. 4,039,936 entitled "INTERLEAVED MAGNETORESISTIVE DISPLACEMENT TRANSDUCERS" and patented to A. L. Jones et al on Aug. 2, 1977. This U.S. Pat. No. 4,039,936 discloses a magnetoresistive displacement transducer arrangement for toothed magnetic members in which each transducer is formed by a pair of interconnected sections spaced from each other by the width of a tooth to provide greater change and improved symmetry in the output signal at the tooth edges. Further, this U.S. Pat. No. 4,039,936 shows in FIG. 4 a bridge circuit arrangement suitable for producing a variable voltage output in accordance with rack tooth detection. In this bridge circuit, one input terminal is connected at the junction between two series-connected equal resistors and the other input terminal is connected at a center tap between two magnetoresistive sections. Suppose that transducers such as those disclosed in this U.S. Pat. No. 4,039,936 are used to form a magnetic rotation encoder in which the two magnetoresistive sections have the same resistance value R. When a voltage V is applied across the input terminals of this magnetic rotation encoder, the variation in the output voltage due to a variation .DELTA.R in the resistance value R of the magnetoresistive sections owing to a variation in the magnetic flux density is approximately given by (.DELTA.R/2R)V, as described in further detail later. Thus, the output variation is only 1/2 of the output variation in each magnetoresistive section due to the resistance variation rate of .DELTA.R/R, and this means that the detection sensitivity of the magnetic rotation encoder is only as high as 1/2 at the most. Further, the magnetic rotation encoder is only capable of generating an output signal having the same frequency as that of the magnetic information or teeth and is not capable of generating an output signal having a higher frequency.